From: Gideon Zweijtzer (gideonz_at_dds.nl)
Date: 2003-11-05 06:47:00
> > Everywhere I look (almost everywhere then) it lists BRK as a > single byte opcode but in operation it is 2. Is there some > official explaination? I notice most assemblers treat it as a > single byte and maybe causing headaches for many users. > Hello Nathan, Of course, only the design team of the 6502 knows for sure what the reason is, but the reason that I found out is that the BRK instruction is just on a 2-byte size location in the instruction map. This is the rule that I use for my own processor implementation. The size of a 6502 instruction is easily derived from the opcode by the following formula: (The result is a two-bit value, where "00" is one byte, "01" is two bytes and "10" is three bytes): Bit1: xxxx11xx or xxx110x1 Bit0: xxxx0xxx or xxx010x1 Regards, Gideon Message was sent through the cbm-hackers mailing list
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